"Prediction of Unsteady Pressure and Velocity Over a Rotorcraft in
Forward Flight ".
N.M. Komerath, D.M. Mavris, and S-G. Liou.
Journal of Aircraft, Vol. 28, No. 8, August 1991, p. 509 - 516.
Abstract: The problem of predicting the periodic three-dimensional vortex-dominated
flowfield around a rotorcraft in low-speed forward flight is studied using
potential-flow methods, whose results are compared with surface pressure
measurements and flow velocity measurements. The test case used is a 2-bladed
teetering rotor above a hemisphere-cylinder airframe in a wind tunnel. The
dominant features of this problem are modeled by a lifting line/lifting
surface rotor model with a free wake distorting in the presence of the airframe.
The airframe flowfield is modeled using a source/doublet panel method. The
instantaneous flowfield is computed at specified intervals of rotor azimuth,
with the unsteady effects of blade motion added to the formulation. As a
simpler alternative, modeling the energy addition at the rotor using actuator
segments led to successful prediction of the time-averaged pressure field,
and the periodic velocity variations along the spine of the airframe. When
a fully unsteady potential formulation is used, however, large differences
appear between measured and computed periodic velocity at the sides of the
airframe. These are due to inadequate understanding of interaction of the
rotor tip vortices with the airframe surface. When measured velocity data
are used in the vortex interaction region, the surface pressure is predicted
quite well even at the sides.